Analysis of a multi-Fano plasmonic split-ring structure using characteristic mode theory for optical applications

Abstract

Multi-Fano plasmonic structures are attractive for sensing and energy harvesting due to their ability to adjust resonances at several wavelengths and the high electric field enhancement. In this paper, the Characteristic Mode Theory (CMT) is used to get a multi-Fano structure, for the first time. For this purpose, the results of CMT in the Heptamer consisting of nanorings, are used. It has been shown that modes can resonate and couple together at the same time if the bending frequency of their eigenvalues is between their resonant frequencies (1470 nm for bending, 1382 and 1600 nm for the two modes) which has not been reviewed anywhere before. Then, to create more couplings between the modes and to produce a multi-Fano structure, the modes with coupling ability and their characteristic current distribution are considered. Some gaps have been created in the location of the smallest currents for increasing the coupling and intensifying the electric field (unlike trial and error methods previously used to improve the performance of the structures), and so, two split-ring Heptamer structures are created as multi-Fano structures. It has been shown that the proposed structure has a significant electric field enhancement of |450|2 which is a good choice for energy harvesting. In addition, these structures can be used for sensors with the most sensitivity of 840 nm / RIU, which has a significant sensitivity than similar structures.